Chilled water systems typically use a primary pump per chiller to pump returning water through the chiller at a relatively low pressure. The returning water is typically at a temperature of about 55.degree. F. (12.8.degree. C.), and its temperature is lowered by the chiller to about 45.degree. F. F (7.2.degree. C.) as it exits the chiller. The primary chilled water pump provides a constant flow and pressure which is matched to the requirements and capacities of the chiller. For example, the primary chilled water pump could produce a flow Q.sub.1 at a pressure H, which satisfies the requirements of its associated chiller, and which, in turn, produces the same flow Q.sub.1 at the chiller's outlet. If the system includes two chillers which are both on-line simultaneously, the total primary chilled water supply flow would be 2Q.sub.1.
The primary chilled water system supplies chilled water to a secondary chilled water system in most installations. A secondary chilled water system has larger pumps which pump the chilled water at a much higher pressure to valves and cooling coils, which make up the load of the chilled water system. If, for example, the chilled water system is designed to cool a high-rise office building, then the secondary chilled water pumps must be able to produce a sufficient pressure H.sub.2 to pump the water to all required locations of that high-rise building.
In some existing chilled water system designs, constant speed pumps have been used for the chilled water pumps. In such systems, if one chiller and one primary chilled water pump are on-line, for example, producing a flow Q.sub.1, then typically one secondary chilled water pump is also running which supplies the system's required flow. If the primary chilled water pumps and the secondary chilled water pumps are not pumping at the same flow rate, which is often the case, then a bypass line must be installed so that excess primary chilled water can flow back into the suction side of the primary chilled water pumps without being circulated throughout the secondary chilled water system. This would occur, of course, in situations where the primary chilled water pump was pumping a flow rate Q.sub.1, which was greater than the flow rate Q.sub.2 then being pumped by the secondary chilled water pump. If, on the other hand, the secondary chilled water pump(s) was pumping at a flow rate Q.sub.3, which was greater than Q.sub.2, then the same bypass line would circulate excess secondary chilled water from the secondary chilled water system return line back into the suction side of the secondary chilled water pump(s).
Many of the systems installed over the last 10-15 years have used variable speed pumps for secondary chilled water pumps. The variable speed secondary chilled water pump allows the overall chilled water system to automatically and efficiently compensate for a varying cooling load by supplying only the required amount of chilled water flow and head (pressure) to satisfy the system demand. The secondary chilled water pumps are sized so that they can provide sufficient chilled water for a maximum cooling demand condition, with the secondary chilled water pumps are running at their maximum speeds. When the system cooling demand is less than maximum, then the secondary chilled water pumps can run at a reduced speed, thereby saving electrical energy.
Since, in such systems, the primary chilled water pumps are constant speed pumps sized to match up with their particular chillers, and the secondary chilled water pumps are variable speed pumps, there typically will be a miss-match between the primary chilled water flow and the secondary chilled water flow required to satisfy the system load. Since it is easier to run a variable speed secondary chilled water pump at a reduced speed than it is to start and stop extra chillers, the primary chilled water system will typically produce more water flow than is required for the secondary chilled water system's needs. That being the case, the bypass line will typically recirculate primary chilled water.
Many of the chilled water systems available at the present time include a package pumping system, in which the primary chilled water pumps and the secondary chilled water pumps are all mounted to a common mounting base. In such systems, it is common to include a suction manifold which receives water from the chillers (primary chilled water supply), and water from the system cooling coils (secondary chilled water return). This water is then directed into the suction side of the various secondary chilled water and primary chilled water pumps. The location where the secondary chilled water return connects into the suction manifold is usually in the form of a "T"-type intersection, in which bypass water (typically excess primary chilled water) collides with the returning secondary chilled water. The combination of bypass water and return secondary chilled water is then directed into the suction side of the primary chilled water pumps.
A major failing of such chilled water systems available at the present time is that the bypass water and the returned secondary chilled water are not blended together properly, thus producing a non-uniform temperature profile along the cross-section of the suction manifold pipe and, in situations where more than one primary chilled water pump is running, one or more of the primary chilled water pumps will receive warmer water at its suction side than the remaining primary chilled water pumps. The use of methods such as having a straight bypass run of at least ten pipe diameters at the bypass line does not solve this problem, because the extra length of straight pipe merely allows the flow of the bypass water to become less turbulent and more laminar. Having laminar flow bypass water does not help to blend that bypass water with the returning secondary chilled water. In fact, the opposite is true. The more turbulent the bypass and returning secondary chilled water, the greater the blending between the two waters to create a more uniform temperature profile along the cross-section of the suction manifold pipe feeding the primary chilled water pumps.